The first portion of these investigations is intended to determine the conditions under which 1) conjugationally transmitted plasmids, 2) donor specific bacteriophage, and 3) temperate bacteriophage can become established and will be maintained in populations of enteric bacteria. Consideration will be given to the population dynamic of bacteria carrying plasmids and the interactions of bacteria with phage in continuous and serial transfer culture and the nature and effects of natural selection on the host, plasmid and phage populations. The second portion of these investigations will be concerned with the exchange of chromosomal genes among clones in populations of enteric bacteria. Estimates will be made of the rates of plasmid or phage mediated gene exchange in experimental populations of E. coli and procedures will be developed for estimating gene exchange rates in natural population of this species. One of the primary goals of these gene exchange studies is to evaluate the relative roles of recombination and clonal or periodic selection in the adaptation and evolution of populations of enteric bacteria. Of particular concern is the relationship between the absolute size of these populations and their "genetically effective" size. The majority of these investigations are to be performed at both a theoretical and an experimental level. The former will involve the development and analysis of mathematical models of populations of bacteria with plasmids and phage and the latter, studies with experimental (Chemostat and serial transfer) populations of E. coli with some of its plasmids and phage.